Wind turbines may use a permanent magnet generator as well as an electrically excited synchronous and/or an asynchronous generator. Generators have in some embodiments one or more stator windings in which a rotating magnetic field of the rotor induces a voltage. Typically, the induced voltage is proportional to the rotational speed of a rotor of the generator and is also proportional to the field of the permanent magnets. Wind turbines may use a permanent magnet generator instead of an electrically excited synchronous or asynchronous generator to improve the system efficiency, in particular at partial load, and, in some embodiments, the annual energy production (AEP) of the wind turbine is increased. Generators have in some embodiments one or more stator windings in which a rotating magnetic field of the magnets, in particular the permanent magnets, induce a voltage.
In typical usage, the generator of a wind turbine is electrically connected to a grid, in particular via inverters. Typically, grid operators require a minimum voltage to be supplied to the grid so that, in case, the generated current is above said minimum voltage, the wind turbine is connected to the grid.
For example, inverters of a wind turbine may only be operated in a predetermined operation range of rotational speed. Thus, during operation, the output voltage of the generator may fit into the input operating range of the inverter or converter. Typically, the output voltage of a generator depends on the rotational speed of the rotor of the generator, in particular in case of a permanent magnet generator. As the wind speed can not be controlled, and thus the rotational speed of the wind rotor of the wind turbine, the time in which the wind turbine may be connected to a grid depends on the wind speed. Further, at high rotational speeds of the wind rotor, high voltages are typically generated by the generator. Typically, the higher voltages the higher may be the iron losses of the rotor.